This invention relates to the continuous measurement of the areal density of a material or object. Any material which tends to attenuate a radioactive source as the material is moved between a radioactive source and a receiver in direct proportion to its areal density, i.e. the density of the material times its thickness. The present invention includes a scintillator for sensing the attenuated radiation which traverses the material and generates light which may be transmitted by a light pipe to an anode of a photomultiplier device for further amplification and processing. The receiver has several embodiments which uniquely utilize scintillating materials and, in some cases, combining them with light pipes for continuous measurement of the areal density of the material throughout a substantial length or width thereof. One receiver embodiment includes a plurality of scintillating optical fibers which may be arranged contiguously and transversely to the radiation source such that light generated by the scintillator embedded in the fiber is piped directly through the fiber to a photomultiplier device for collection and amplification. If a multi-anode photomultiplier device is utilized, the areal density of the material at specific locations thereof may be conveniently collected on a continuous basis. When position data is not desired, a scintillator may be directly coupled to the photomultiplier device. In still other embodiments, a scintillator film may be utilized with optical fibers as light pipes only for transmitting the photons from the scintillator film to the photomultiplier device.
The subject invention may be utilized in several different formats and applications, depending upon the desires of the user. Several of these are disclosed and described herein with specific features adapted for implementing the invention to the specific task at hand. However, the scope of the invention should be considered to be broader than the scope of the particular examples and applications disclosed herein which are intended to be merely exemplary thereof.
One specific application of this invention relates to the continuous measurement of the areal density of a traveling web, and in particular, to the determination of the areal density thereof simultaneously at multiple points which substantially cover the entire width thereof. The preferred embodiment discloses an application of this invention to the problem of determining the basis weight of a paper web, however, the invention may be used to determine the areal density Of many different kinds of materials, such as metal, woven materials such as cotton and synthetic textiles, and non-woven materials such as plastics, etc. Thusly, although the term web is used throughout, it should be understood that it refers to any of these materials.
Presently, the basis weight of a web of paper is determined using a single source which emits Beta or Gamma radiation particles. The radiation passes through the paper web and is measured by a detector on the other side of the web of paper. As it passes through the web, the beam of radiation is attenuated. The attenuation is proportional to the density and thickness of the web, as expressed in terms of its areal density. Thus, the signal received is proportional to the basis weight of the paper web.
The source that is used is generally quite small, only an inch or two in diameter. Consequently, the detector must be mobile to measure the full width of the paper web. Presently, the detector is mounted on an endless belt and travels to and fro across the width of the paper. Thus, at any one time, the information received is only determinative of the areal density of the paper at a single point. To determine the basis weight of the paper, thousands of measurements have to be taken as the device scans across the paper. Because the paper is moving past the measuring device quickly, i.e. up to 7,000 FPM for newsprint, the time spent by the device in measuring a specific part of the web, in the cross direction, is limited. Further, because only an average basis weight, over many thousands of feet of paper, can be determined, it is not possible to use the present scanners to investigate short time span variables which affect basis weight and therefore affect product quality.
Another example of a prior art device is shown in U.S. Pat. No. 3,868,510 entitled Method for Sensing Profile of Sheet or Plate Materials. This device discloses a radiation thickness detector which utilizes a radiation source which is physically moved transversely to a moving sheet of steel. The detector comprises a flat detector plate made from a scintillator with a number of light pipes secured to the scintillator for routing light generated therein to a single anode photomultiplier such as a photoelectron multiplication tube. This device, as with the other prior art device disclosed herein, measures the relative thickness of the moving web at only one point, that point being where the source is positioned. Therefore, this device provides a position sensitive measurement of the thickness of the web also.
In accordance with the invention, one of the preferred embodiments discloses a paper basis weight detector for simultaneously determining the basis weight of a web of paper at multiple points across the width thereof. The detector includes a stationary radioactive source which emits radiation which passes through the paper web. A stationary receiver detects the radiation which passes through the web and converts the radiation to light of a known wavelength. A multi-anode photomultiplier tube (or array of photomultiplier tubes) optically connected to the receiver collects the light which is generated by the receiver. The output of the photomultiplier tube is directed to an analyzer which counts the light photons generated and thereby determines the basis weight of said paper webs.
The receiver includes a plurality of scintillating optical fibers formed into an array. The fibers preferably extend transversely to the direction of motion of said paper web. The array is approximately 2-3 cm in width and 15 cm in length. The array, however, may be of an endless variety of dimensions. The receiver preferably includes a plurality of such arrays which, together, extend the width of the paper.
The fibers of the array extend from the detection region to the photomultiplier tube. The scintillating fibers from each array may be joined into a single non-scintillating fiber optic element which is connected to the photomultiplier tube. Preferably, each array of fiber optics is connected to a different anode to allow for discrete measurement at multiple points across the width of the paper web.
The receiver may alternately comprise a scintillator and non-scintillating optical fibers connected thereto. The scintillator may include a scintillating or phosphor screen to which the fibers are connected in an array. Alternatively, the scintillator may include a plurality of discrete scintillating elements, there being one such element at an end of each said fiber. Preferably, each array of fiber optics is connected to a separate anode of the photomultiplier tube to allow simultaneous measurement at multiple points across the paper web.
The detector preferably can detect a wide range of radiation particles, and in an experimental prototype can detect, e.g., approximately 5.5.times.10.sup.6 Beta and related radiation particles/second. To accomplish this, the source preferably includes a source of Strontium-90, Cesium-137, or other similar radioactive or X-ray emitting source. The source strength used depends upon the specific measurement intended. The Beta particles or X-rays emitted by the source may, optionally, be passed through a collimator.
Still another specific application of the subject invention relates to the continuous measurement of the fill level of a beverage can as a plurality of beverage cans are conveyed therepast. In this application, a linear radioactive source is positioned directly opposite a receiver, and both are spaced apart to provide a path for the plurality of containers as they are conveyed therethrough. A typical example for this specific application would include the detection of a fill level of beer or soft drink in an aluminum can. The receiver may be comprised of an array of scintillating fibers, with the array being oriented either vertically or horizontally. Also, the array may be positioned along the entire height of the container, or may instead be arranged to cover only that upper portion of the container which might be expected to be partially empty. Additionally, the horizontal array of fibers may be grouped and separately coupled to individual anodes of a multi-anode photomultiplier tube, or other photomultiplier device, such that a more accurate indication of the actual fill level may be detected. Alternately, either the vertical or horizontal array of scintillating fibers may be connected to a single anode photomultiplier tube and the relative strength of the signal would be representative of the level of fill in the particular container being sensed. In this particular embodiment of the receiver, scintillating fibers may be used, or, as suggested in other embodiments of other examples herein, a scintillator sheet with optical fiber may be used, or portions of scintillating fiber integrally joined to optical fiber. In still a third embodiment of this specific application, the receiver may be comprised of a plastic scintillator sheet which is directly coupled to a single anode photomultiplier tube, thereby eliminating the use of any optical fibers, scintillating or non-scintillating. In this embodiment, the relative strength of the signal would be an indicator of the fill level of fluid in the container.
One of the objects of the present invention is to provide a radioactive areal density detector with a receiver having a scintillator for converting radiation which traverses an object or material whose areal density is to be detected into a light signal representative thereof. A photomultiplier device may be conveniently coupled to the scintillator and amplifies the light signal for further use. The scintillator may be comprised of either scintillating fibers, or a scintillator material. Optical fibers, if used, conveniently light pipe the photons generated by the scintillator to the photomultiplier device. A single or multi-anode photomultiplier device can provide either one signal, or multiple signals, representative of areal densities at different physical locations throughout the material or device being sensed. The scintillating fibers may be arranged, horizontally, vertically, or in other arrangements as suits the particular application. Specific embodiments representing applications of the present invention in solving particular problems incorporate additional inventive features.
Another object of the present invention is to provide an apparatus which can simultaneously and continuously, in addition to very quickly, determine the thickness of a traveling web at multiple points across the width thereof. Such cannot currently be done with moving scanners.
Another object is to provide such an apparatus which will provide basis weight measurements of a paper web at much shorter time intervals than presently possible.
Another object is to provide such an apparatus which can be situated either at the web end or the dry end of a web of paper.
Another object is to provide such an apparatus which will produce an accurate weight profile of the full width and length of the web.
Another object of the present invention is to provide an apparatus which can continuously, in addition to very quickly, determine the fill level of a fluid in a container. The apparatus may include a linear radioactive source and a receiver mounted in spaced apart relationship such that the plurality of containers may be conveniently conveyed therebetween.
Another object of the present invention is to provide such an apparatus with a receiver incorporating scintillating fibers arranged in an array which is either vertically or horizontally situated with respect to the containers.
Another object of the invention is to provide such an apparatus which may detect the fill level in the container based on the relative strength of the radiation which traverses the container.
Another object of the invention is to provide such an apparatus which will provide a direct physical measurement of the fill level of fluid in a container by utilizing groups of scintillating fibers arranged horizontally with the output of each group being separately collected and amplified to provide separate signals.
These and other objects will become apparent to those skilled in the art in light of the following disclosure and accompanying drawings.